A micromirror array has been developed as an image-forming optical element for image-forming three-dimensional or two-dimensional objects, video pictures and the like in space. The micromirror array includes a substrate (base) constituting an element surface of an optical element, and a multiplicity of unit optical elements formed in an array on the substrate and each having “two mirror surfaces orthogonal to each other” (a pair of adjacent light reflecting surfaces constituting a right-angled corner; that is, a corner reflector) (see PTL 1 and PTL 2).
The present applicant proposes a display device (video picture display device) including such a micromirror array and configured such that a display P displaying a video picture from which a projected spatial image I′ is made is housed in an attitude inclined at a predetermined angle (α=30° to 90°) with respect to an image-forming optical element (micromirror array M) in a case 20 that is like a dark box, as shown in FIG. 13, in Japanese Patent Application No. 2013-43074.
This display device uses a function (image-forming function) such that light incident on one surface (lower surface as seen in the figure) of the micromirror array M is reflected twice between a pair of light reflecting surfaces constituting each unit optical element (corner reflector) when passing through the array M, so that the light reflected twice (passing light) forms an image in a spatial position symmetrical with respect to the plane of an element surface Q on the other surface side (upper surface side as seen in the figure) of the array M. Thus, a mirror image reversed image of an image I, an object or the like placed on the one surface side of the micromirror array M is image-formed as an aberration-free real image (spatial image I′) in space on the other surface side of the array M (space on the observer H side).
Also, as shown in FIGS. 14A and 14B, a micromirror array M1 (with reference to FIG. 14B) is used as the image-forming optical element in the display device (see PTL 3). In the micromirror array M1, two optical elements 21 and 21′ (with reference to FIG. 14A) having a plurality of parallel linear grooves 21g and 21′g spaced at predetermined intervals and formed by dicing using a rotary blade in the front surfaces of flat-shaped transparent substrates (21 and 21′), respectively, are laid one on top of the other, with one of the optical elements 21 and 21′ rotated horizontally through 90 degrees.
When the micromirror array M1 is viewed in the direction of the front and back surfaces of the substrates (in a vertical direction as seen in the figures), “corner reflectors” are formed respectively at the intersections (points of intersection of a lattice) of a first group of parallel grooves 21g and a second group of parallel grooves 21′g which are orthogonal to each other as seen in plan view. The corner reflectors are comprised of light-reflective vertical surfaces (wall surfaces) of the first group of parallel grooves, and light-reflective vertical surfaces (wall surfaces) of the second group of parallel grooves. In addition to the aforementioned example in which the substrates 21 and 21′ are stacked together by bringing the front surfaces 21a and 21′a of the respective substrates 21 and 21′ into abutment with each other, there are other cases in which the substrates 21 and 21′ are stacked together by bringing the back surfaces 21b and 21′b of the respective substrates 21 and 21′ into abutment with each other and in which the substrates 21 and 21′ are stacked together by bringing the front surface 21a and the back surface 21′b thereof into abutment with each other and by bringing the back surface 21b and the front surface 21′a thereof into abutment with each other.